In the fabrication process for a semiconductor device, numerous processing steps, i.e., as many as several hundred, must be performed on a semiconducting wafer to form the device. In the numerous processes, a wafer is frequently positioned on a rotating wafer chuck, or a vacuum chuck, for carrying out the specific process. Among such processes are the photoresist coating process, the photoresist developing process and an edge bead rinse process for removing an edge bead.
In a typical photoresist coating process, a small amount of a photoresist liquid is dispensed onto a wafer that rotates at high speed from a dispensing nozzle positioned at the center of the wafer. Since only a very thin coating of the photoresist material is normally required, the amount of the liquid photoresist material dispensed is small. When a liquid dispensing nozzle is not perfectly centered in relation to a rotating wafer, incomplete coverage or even voids on the wafer surface can occur. Such poor coverage of a photoresist coating on a wafer surface results in a high scrap rate of the devices formed on the wafer or even the scrap of the whole wafer.
After the photoresist coating process, a wafer is imaged in a stepper to reproduce circuits desired on the wafer. A liquid developer material is then dispensed onto the surface of the wafer in a technique similar to that used in dispensing the photoresist liquid. A perfectly centered dispensing nozzle for the developing liquid is crucial to the successful developing of the photoresist layer in order to reproduce the circuits.
After a photoresist material is coated, exposed and developed on the wafer surface, an edge bead rinse (EBR) process is frequently performed before the wafer can be further processed. This is due to the fact that, in most processing chambers, a clamp ring is used to hold the wafer down on a platform during a chemical or physical process. A peripheral edge on the top surface of the wafer that is overlapped by the clamp ring must therefore be cleaned of the photoresist material so that no photoresist material could crack under the clamp ring and thus causing serious contamination problems. The edge bead rinse process is an important step that must be carried out after a photoresist coating and developing process.
The wafer processing steps that involve the photoresist coating, developing and edge bead rinsing can be performed in a variety of commercially available process machines. One of such process machine is a WAFER-TRACK.RTM. machine for loading/unloading wafers into and out of various process chambers for photoresist coating, for photoresist developer coating and for edge bead rinsing. In a conventional wafer loading process, a main arm delivers a wafer into a process chamber through a slot opening. After a wafer is loaded on top of a vacuum chuck, the main arm withdraws from the slot opening. The movement of the main arm is controlled by a process controller through digital inputs by a machine operator. Conventionally, the only way for checking whether a wafer has been positioned, or centered, properly on a vacuum chuck is to rotate the wafer by hand. If the wafer is not positioned concentric with the vacuum chuck as indicated by an off-centered rotation of the wafer, the machine operator inputs a new set of digital data into the process controller based solely on his experience. The process is normally repeated several times before the movement of the main arm in positioning the wafer is properly calibrated. It is a trial and error process which requires a high skill level on the part of the machine operator. The procedure is therefore both time consuming and highly operator dependent.
In a conventional edge bead rinse process conducted in an edge bead rinse chamber, a nozzle assembly controlled by a digital step motor is frequently used to wash the edge bead on a wafer surface. Each bit or pulse movement by the digital step motor causes a displacement of the nozzle head by a distance of approximately 0.025 mm. For an eight inch wafer, a width of 3 mm.+-.0.5 mm on the outer edge of the wafer is normally required for edge bead removal.
When a photoresist coated wafer is placed on a vacuum chuck for the edge bead rinse process, the centering of the wafer in relation to the vacuum chuck is very important. Improperly centered wafer results in an unevenly washed wafer edge with one side of the wafer having excessive photoresist coating and the other side of the wafer having insufficient photoresist coating. To prevent the uneven rinse of a wafer in an edge bead rinse process, a machine operator must provide data input to the main arm of the robot for positioning the wafer before a correct position can be found. The trial and error process is both labor and time consuming.
A conventional method for adjusting an edge bead rinse process is shown in FIG. 1. A wafer 10 which has a photoresist coating layer 12 on top was processed through an edge bead rinse chamber. After the edge bead rinse process was carried out, as shown in FIG. 1, the edge bead rinse process resulted in an off-centered coating pattern in that the washed edge A is wider than the opposite, washed edge B. In a conventional calibration method, a straight ruler 14 is used to measure the distance A and B at the two opposite ends of the wafer which appears to have a maximum and a minimum washed edge. After the values of A and B are measured by the straight ruler 14, the equation of C=(B-A)/2 is used to determine the magnitude of eccentricity (the distance between the centers for the two circular areas). The necessary digital input into the process controller for the robot (or the main arm) can then be calculated by using the "distance per pause" value D. For instance, when the magnitude of eccentricity is determined to be +0.25 mm, and the distance per pause value D for the robot arm is 0.025 mm/pulse, then the unit position of the robot arm should be adjusted by 10 pauses toward the positive direction. This is a slow and painstaking process in that the wafer must be carefully measured by a steel ruler and calculations must be carefully conducted. Several corrections are sometimes required to correctly center the wafer.
It is therefore an object of the present invention to provide an apparatus for measuring the eccentricity of a photoresist coating on a wafer that does not have the drawbacks or shortcomings of the conventional apparatus.
It is another object of the present invention to provide an apparatus for measuring eccentricity between two circular members attached together that is easy to operate and produces reliable results.
It is a further object of the present invention to provide an apparatus for measuring eccentricity between two circular members attached together by utilizing a vernier measuring device including a main scale and a vernier scale.
It is another further object of the present invention to provide an apparatus for measuring eccentricity between two circular members attached together by utilizing a vernier measuring device and a mounting block which has a V-shaped top surface for positioning the circular members.
It is still another object of the present invention to provide an apparatus for measuring eccentricity between a wafer and a coating layer on the wafer wherein the coating layer has a diameter that is slightly smaller than a diameter of the wafer.
It is yet another object of the present invention to provide an apparatus for measuring eccentricity between a wafer and a coating layer on the wafer by fixing a main scale of a vernier measuring device on a mounting block in such a way that a zero setting on the main scale coincides with the center of the wafer while the zero setting on the vernier scale coincides with the center of the coating layer.
It is still another further object of the present invention to provide a method for measuring eccentricity between a wafer and a coating layer on the wafer by mounting the wafer on a V-shaped mounting block and then aligning the zero setting on the main scale with the center of the wafer, the zero setting on the vernier scale with the center of the coating layer and determining the distance between the two zero settings.
It is yet another further object of the present invention to provide a method for measuring eccentricity between a wafer and a coating layer on a wafer by fixing the main scale of a vernier measuring device on a mounting block equipped with a V-shaped notch in a top surface and then positioning the wafer in the V-shaped notch such that the center of the wafer forms a line with the tip of the V which is perpendicular to a top surface of the mounting block.